India, Sept. 25 -- A farmer in Maharashtra wants to know how much rain will fall over the next three to ten days of the kharif season. An engineer wants to know how intense the monsoon downpours can become in 2070, to build storm drains now that must last for decades. Science can answer these two questions with different levels of confidence, with implications for how South Asia should prepare.

The monsoon depends on the temperature contrast between land and ocean, sea surface temperatures in the Pacific and Indian Oceans, the Himalayas steering winds, and aerosols from air pollution over the Indo-Gangetic Plain that reflect sunlight and can serve as cloud seeds. Monsoon rain is produced by convective clouds a few kilometres across, whil...